A White Paper is a paper that concisely sets out the government's position on a specific issue. Part of this white paper includes, as mentioned in the manifesto, a promise to give councils powers to intervene where developers do not act on their planning permissions.
The entirety of the reforms have not yet been implemented. Theresa May did commit, in her 2017 Conference Speech, that councils would receive powers to ensure that developers build on lands they have won planning permission for.
Category: Social and Welfare
Source: Manifesto - Page 70
Reference 1: Housing White Paper
Reference 2: Housing consultation
Reference 3: Conference Speech